Not much difference from what I see.
Mk 6 has different touchpad from mk5 and I am pretty sure it will boot UEFI, but it is not mentioned anywhere.
Mk7,mk8 gave UEFI switch in bios.
If you want a mk6, let me know. I have one together and one in a box.
Mk6 has USB3, way better GPU, more frugal yet faster CPU (Sandy Bridge is trash power consumption wise), Ambient Light Sensor and that's it. Mk7's are not worth the money and the 19 Mk8 is a straight up scam, shit-tier CPU for the same price, no thank you very much Panasuckit!
Also, UEFI = NSA/CIA/idefk backdoor risk, you'll never know what they put in it.
Then again, if you want maximum freedumbs a ThinkPad T60 with Libreboot (maintainer is a batshit insane tranny though) and Gentoo Hardened should get you maximum freedom, hardware and software wise.